Kaposi's sarcoma is a highly vascular skin lesion consisting of clonal and polyc!onal expansions of endothelial cell derived spindle cells that carry multiple copy episomal genomes of the newest human herpesvirus to be discovered, referred to as KSHV or HHV8. Seropositivity for KSHV infection in the USA, Europe and Asia is low, but the risk factor for KS in male homosexual AIDS patients in increased 20,000 fold, with up to 20 percent developing lesions and 50 to 85 percent becoming infected. KS is now also the most commonly encountered tumor in Sub-Saharan Africa, where HIV has spread into a population that was nearly ubiquitously infected with KSHV, as well as in renal transplant patients in other populations with relatively high endemic KSHV infection rates. The DNA genome of KSHV differs from that of EBV by lacking all of the latent state EBNA genes, but instead encodes a set of captured cellular genes, including cytokines, chemokines, cyclin and chemokine receptor genes, that have novel anti-apoptotic and angiogenic properties. In this competitive renewal, we propose to continue our studies of the early stages of KSHV lytic cycle infection in both PEL lymphocytes and primary dermal microvascular endothelial cells. These experiments will focus on the viral and cellular proteins involved in the switch from latent to lytic infection, the earliest stages of transcriptional regulation, and the initiation of lytic DNA replication, as well as the regulatory interactions between transcription control and DNA replication. The three Specific Aims encompass; firstly, examination of the patterns of viral and altered cellular gene expression in lytic infection in vascular endothelial cells; secondly, studies of the cis-acting domains and trans-acting factors that participate in the switch from latent to lytic cycle infection, with emphasis on the role of the RTA(ORF50) transactivator and differential promoter regulation; thirdly, analysis of the mechanism and control of KSHV DNA replication, especially related to identifying and studying ori-Lyt and the predicted origin binding protein, as well as the role of ORF-K8 C/EBP and cell cycle controls.